Effect of Temperature on Organic Carbon-texture Relationships in Mollisols and Aridisols1

P. A. MCDANIEL AND L. C. MUNN2

ABSTRACTData from 143 grassland soils of Montana and Wyoming clas-

sified as Mollisols or Aridisols were analyzed to determine the effectof soil temperature regime on the relationship between organic car-bon (C) and texture. Using existing data, average weighted organicC content to a 40-cm depth was calculated for each soil and corre-lated with several variables, including elevation, weighted clay andsand contents to a 40-cm depth, and sand/clay ratio. For all soils,organic C was significantly correlated with sand (r = — 0.27) butnot with clay (r = 0.01). When soils were grouped by combinationsof temperature regime and classification at the order level, organicC levels were highest in cryic soils (Cryoborolls) and lowest in mesicAridisols. Correlations of organic C with sand and clay contentswere highest for mesic Mollisols and mesic Aridisols and decreasedas temperature regimes became colder. Sand/clay ratio was signif-icantly correlated with organic C in mesic soils only, and thereforemay not be useful in distinguishing between Typic and Borollicsubgroups of frigid Aridisols in Soil Taxonomy. Results suggest thatsand and clay contents have limited value in predicting organic mat-ter levels in cold arid and semiarid grassland soils.

Additional Index Words: native range, grassland, frigid, cryic,mesic, sand/clay ratio, Borolls, Cryoborolls.

McDaniel, P.A., and L.C. Munn. 1985. Effect of temperature onorganic carbon-texture relationships in Mollisols and Aridisols. SoilScT. Soc. Am. J. 49:1486-1489.

IT HAS LONG BEEN RECOGNIZED that organic mattercontent of soils is influenced by soil texture. With

other factors being equal, clay content of a soil is pos-itively correlated with organic matter content and, assand content increases, organic C levels generally de-crease (Allison, 1973; Brady, 1984; Foth, 1984; Ste-venson, 1982). However, little research has been doneto examine the effect of temperature on this relation-ship in soils that have formed under otherwise similarconditions.

Soil Taxonomy uses organic C as a denning char-acteristic of the mollic epipedon and the Mollisol or-der (Soil Survey Staff, 1975). In addition, the rela-tionship between soil texture (expressed as a sand/clayratio) and organic C content is used to distinguishTypic subgroups of Aridisols from Borollic (frigid) and

1 Contribution of the Wyoming Agric. Exp. Stn., Univ. of Wyo-ming, Laramie, WY 82071. Journal Series no. JA 1353. Received8 Apr. 1985. Approved 2 July 1985.2 Former Research Associate, Plant Science Dep., Univ. of Wy-oming, Laramie; now Instructor, Dep. of Soil Science, North Car-olina State Univ., Raleigh, NC 27695-7619; and Assistant Professorof Soil Science, Plant Science Dep., Univ. of Wyoming, Laramie,WY 82071.

Ustollic (mesic) intergrades. As sand/clay ratio de-creases, more organic C is required for Borollic or Us-tollic subgroups.

Several reasons for the strong correlation betweenorganic matter and soil texture have been suggested.As sand content decreases and clay content increases,there is often an increase in the available water hold-ing capacity of a soil. In semiarid and subhumid cli-mates, this generally results in increased vegetativeproduction and subsequently higher levels of soil or-ganic C (Munn et al, 1978). Correlation may also bedue in part to adsorption of humic substances to clayparticles. Stabilization or protection of soil organicmatter by organo-mineral complexes has been re-ported by many researchers. Much of this work hasbeen reviewed by Bremner (1965) and Greenland(1965). It has also been shown that decomposition ofsoil organic matter may be retarded by adsorption andsubsequent deactivation of extracellular carbon de-grading enzymes by clay particles (Bremner, 1965).

Anderson and Paul (1984) found that organic C as-sociated with fine clay in a cultivated Haploboroll hada relatively young radiocarbon age and was apparentlyprotected against further rapid decomposition. Theysuggested this stabilizing mechanism could lengthenthe turnover time of otherwise labile humic materialsfrom a period of days to months, years, or decades.

As much as 50% of the total humus in some grass-land soils may be associated with clay (Anderson, 1979;Paul and van Veen, 1978). Stevenson (1982) suggestedthat organic matter accumulation reaches equilibriumfaster in finer-textured soils, due in part to mineralprotection of humic materials.

Nichols (1984) reported a significant relationshipbetween clay and organic-matter content of Mollisolsand associated soils in the southern Great Plains. Hefound a correlation coefficient of 0.86 which was sig-nificant at the 1% level.

The purpose of this study was to examine the re-lationship between organic C and the sand and claycontents for a number of grassland soils that are ex-tensive in Montana and Wyoming. By grouping soilsby temperature regime we were able to determine theeffect of soil temperature regime on this relationship.

MATERIALS AND METHODSSoil data for 143 pedons from Montana and Wyoming

were obtained from the Bureau of Land Management inMontana (McDaniel et al., 1982), Soil Survey InvestigationsReport no. 32 (Soil Conservation Service, 1978), and Munn

MCDANIEL & MUNN: TEMPERATURE AND ORGANIC C-TEXTURE RELATIONSHIPS 1487

Fig. 1. Map of Montana and Wyoming showing county locations(shaded) of soils selected for use in this study.

(1977). An attempt was made to select soils that were ex-tensive throughout the study area and had developed in avariety of parent materials under native grass vegetation.County locations for soils used in this study are shown inFig. 1.

Elevations ranged from 845 to 2535 m (2400-8160 ft).Soils had mesic, frigid, or cryic temperature regimes andaridic, ustic, or udic moisture regimes. Temperature regimesfor Montana soils were obtained from existing series de-scriptions or estimated using site characteristics (Munn andNielsen, 1978). Temperature regimes for all Wyoming soilswere obtained from Soil Survey Investigations Report no.32 (Soil Conservation Service, 1978). Classifications for thesesoils included both Typic and Aridic subgroups of Mollisolsand Typic, Borollic, and Ustollic subgroups of Aridisols.

All soils were well-drained, present on relatively undis-turbed sites, and under predominantly native vegetation.Vegetation was extremely variable among the sites, reflect-ing the wide range of climatic and edaphic conditions. Plantcommunities at all sites contained a significant grass com-ponent. Typical grasses present on cooler, moister sites in-cluded Festuca idahoensis Elmer (Idaho fescue) and F. sca-brella Torr. (rough fescue). On warmer, drier sites Stipacomata Trin. & Rupr. (needle and thread) and Boutelouagracilis (H.B.K.) Lag. (blue grama) were common. Agropy-ron spicatum (Pursh) Scribn. & Smith (western wheatgrass),A. smithii Rybd. (bluebunch wheatgrass), and Koeleria cris-tata Pers. (prairie junegrass) were also present at many sites.Artemisia tridentata Nutt. (big sage) was the dominant shrubat most sites. Complete vegetation descriptions are given byMcDaniel et al. (1982) and Munn et al. (1978).

Weighted average organic C content to a 40-cm depth wascalculated for each soil using the following equation:org C(%)=

[org Ci thickness i Dbl + + org Cn thickness,, Dbn][thickness] Dbl + . . . + thickness,, Dbn]

In this formula, n is the number of subhorizons in thesoil to a 40-cm depth; org Ct is the organic C percentage in

Table 1. Correlation of weighted organic C to 40 cm and selected______properties for 143 Mollisols and Aridisols.______

Property r

Elevation 0.55**Weighted avg. sand content to 40 cm -0.27**Weighted avg. silt content to 40 cm 0.26**Weighted avg. clay content to 40 cm 0.01Weighted avg. sand/clay ratio to 40 cm -0.14

** Significant at the 1% level.

Table 2. Average weighted organic C content to 40 cm andcorrelation with clay, sand, and sand/clay for soils

grouped by order and temperature regime.

Weighted

Correlation with organic Cto 40 cm

Sample setavg org. C Clay to Sand to Sand/clay

n to 40 cm 40cm 40cm to 40 cm

Mollisols (mesic)Mollisols (frigid)Mollisols (cryic)Mollisols (all samples)Aridisols (mesic)Aridisols (frigid)Aridisols (all samples)

6414289114354

gkg-10.218.525.921.4

6.212.211.0

0.81*0.33*0.260.24*0.65*

-0.170.08

-0.97**-0.36**-0.30-41**

-0.92**0.03

-0.24

-0.94**0.040.28

-0.22*-0.81**

0.01-0.33*

*,** Significant at the 5 and 1% levels, respectively.

the first subhorizon; thickness! is the thickness of the firstsubhorizon; Dbl is the bulk density of the first subhorizon;etc. Organic C contents were expressed to a 40-cm depth toallow comparison with criteria used in Soil Taxonomy (SoilSurvey Staff, 1975). Weighted average clay and sand con-tents to a 40-cm depth were also calculated in the same wayand a sand to clay ratio was determined using Method B ofNettleton and Brasher (1979), corrected for bulk density.

Simple linear regressions of weighted organic C to 40 cmvs. weighted clay, silt, and sand contents to 40 cm, elevation,and sand/clay ratio were performed for all soils. Similaranalysis was performed for soils grouped by combinationsof temperature regime and classification at the order level.

As defined in Soil Taxonomy, frigid soils are those havinga mean annual soil temperature (MAST) between 0 and 8°C.Cryic soils also have a MAST between 0 and 8°C, but inaddition, have a mean summer soil temperature, for well-drained soils, of <15°C if there is no O horizon present or<8°C if there is an O horizon. Mesic soils have MASTbetween 8 and 15°C.

RESULTS AND DISCUSSIONFor all soils, organic C is most strongly correlated

with elevation (r = 0.51) and sand content (r = —0.27,Table 1). No significant correlation was found be-tween organic C and sand/clay ratio or clay content(r = —0.14 and r = 0.01, respectively). When soilswere grouped by temperature regime and classificationat the order level, mean organic C content is higherin soils classified as Mollisols. Organic C also in-creases within both the Aridisol and Mollisol ordersas temperature regimes become colder (Table 2).

Organic C content is best correlated with sand con-tent in mesic Mollisols and mesic Aridisols (r = —0.97and r = —0.92, respectively). Correlation becomesprogressively poorer for frigid Mollisols, cryic Molli-sols (Fig. 2), and frigid Aridisols.

An identical trend is also seen in the relationship

1488 SOIL SCI. SOC. AM. J., VOL. 49, 1985

o:

o< a

MESICr=.8ln=6

15 20 25 30CLAY (%)

o>

§ 10ccO 9O

I

35 40 45 50 55 60 65 70 75 80SAND (%)

70

T~ 60jcen"-" 50Om§40

O2 30

O 20

10

FRIGIDr = .33n = 4l

10 15 20 25 30 35 40 45 50CLAY '(%)

70

-P'60

' 50

40

^30

O 20

10

FRIGIDr = -.36

20 30 40 50SAND (%)

60 70

50

'0.40_aeCT

Om

OO

CRYICr = .!6n=42

15 20 25 30 35 40 45 50 55CLAY (%)

50

'0.40

o>

CD30

OO

20

CRYICr"-.30

10 20 30 40 50 60 70SAND (%)

Fig. 2. Effect of temperature regime on the relationship of organic C with clay and sand content for Mollisols used in this study. Valuesrepresent weighted averages calculated to a 40-cm depth.

MCDANIEL & MUNN: TEMPERATURE AND ORGANIC C-TEXTURE RELATIONSHIPS 1489

between organic C and clay content. Strongest corre-lations are seen in mesic Mollisols and mesic Aridisols(r — 0.81 and r = 0.65, respectively). Again, corre-lation becomes progressively poorer for frigid and cryicgroupings (Fig. 2.).

Sand/clay ratios for mesic Mollisols and Aridisolsshow significant correlation with organic C (r = —0.94and! r = —0.87, respectively). However, for frigid andcryic soils, no significant relationship is seen. Thus, itappears the sand/clay ratio used in conjunction withweighted organic C content may provide a good basisfor distinguishing between Ustollic and Typic Aridi-sols. In these mesic soils, a significant relationship doesexist between these two properties. However, becausethe relationship does not appear to be as strong infrigid soils, its use in distinguishing between Borollicand Typic Aridisols may not be valid.

Poor correlations observed for frigid Aridisols maybe due, in part, to the inclusion of soils in this groupwhich may actually have cryic temperature regimes.Because Soil Taxonomy does not recognize the com-bination of aridic moisture regimes and cryic temper-ature regimes, all Aridisols having a mean annual soiltemperature <8°C were assumed to be frigid. How-ever, there is evidence that some Aridisols of the west-ern USA do have cryic temperature regimes (Jensen,1984).

Results of this study suggest that the influence ofsoil texture on organic C content of Mollisols and Ar-idisols is more pronounced in soils with mesic tem-perature regimes than in soils with frigid and cryictemperature regimes. Although relatively few mesicsoils were used in this study, results are consistentwith those of Nichols (1984). He reported high cor-relation (r = 0.86) between clay and organic C contentin soils having mesic or thermic temperature regimes.

Little relationship is seen between texture and or-ganic C in frigid and cryic grassland soils. This mayindicate that factors responsible for this relationshipin mesic and thermic soils are less important in coldersoils. Water holding capacity and protection of or-ganic matter by adsorption to clay particles are ap-parently less significant in regulating organic C levels.Temperature, rather than moisture, appears to be-come the limiting factor in decomposition of organicresidues in these colder soils. Also, organic matter maybe able to persist in cold soils without the stabilizinginfluence afforded by adsorption to clays. This clay-induced stabilization would likely be increasingly moreimportant in maintaining organic C levels as soils be-come warmer.

CONCLUSIONOrganic C is significantly correlated with sand/clay

ratio only in mesic grassland soils. For frigid and cryicsoils, correlation was not significant and therefore maynot be useful for classification purposes. Results in-dicate that sand and clay contents have limited valuein predicting organic C levels in cold arid and semi-arid grassland soils.